1. Field of the Invention
This invention relates to turbomachinery and is particularly directed to a blade assembly including a ceramic jacket as thermal protection for blades operating at high-temperatures.
2. Description of the Prior Art
In order to improve the performace and fuel economy of turbomachiney, such as pumps or turbines, it has been proposed to operate the turbines at elevated turbine inlet temperatures. Inlet temperatures above 2400.degree. F. are theoretically desirable. However, such temperatures are well above the operating capabilities of even the the most advanced high-strength metals unless complex and costly cooling methods are applied to the blades' exterior surfaces.
Blades comprising high-temperature ceramics have exhibited great potential for fulfilling the goal of accommodating high turbine inlet temperatures without requiring the use of complex surface cooling methods. However, ceramics are brittle and have little capacity for withstanding mechanical or thermally induced tensile stresses. Thus, efforts continue in an attempt to overcome the aforementioned difficulties when utilizing ceramic material in conjunction with high-strength metals in a blade assembly.
One approach is described in U.S. Pat. No. 4,563,128 to Rossmann which discloses a turbine blade suitable for use under super-heated gas operating conditions. Each blade includes a hollow ceramic blade member and an inner metal support core extending substantially radially through the hollow blade member and having a radially outer widened support head. The design of this turbine blade is configured such that radially inner surfaces of the head are inclined at an angle to the turbine axis so as to form a wedge or key forming a dovetail type connection with respectively inclines surfaces of the ceramic blade member. In a preferred embodiment, the turbine blade according to the invention is one with air cooling. For this purpose, the support core comprises several cooling air channels running lengthwise, radially through the blade.
While alleviating certain problems inherent with compressive stress, this design which incorporates the cooling channels or ducts require prohibitively large volumes of cooling air in order to be effective.
An alternative arrangement in the prior art is exemplified by the device taught in U.S. Pat. No. 4,519,745 to Rosman et al, wherein a ceramic blade assembly including a corrugated-metal partition is situated in the space between the ceramic blade element and a post member. The corrugated-metal partition forms a compliant layer for the relief of mechanical stresses in the ceramic blade element during aerodynamic and thermal loading. In addition, alternating cooling channels are juxtaposed between the ceramic blade element and the post member for directing cooling fluid thereover. A second set of passages being adjacent to the interior surfaces of the ceramic blade element are closed off for creating stagnant columns of fluid to thereby insulate the ceramic blade elements from the cooling air. This design, however, attains a less than desired performance under high-temperature operations.
Another significant disadvantage of a blade constructed according to the prior art is that the ceramic blade element is structurally retained without satisfactory means for dampening vibration or relieving aerodynamically induced stresses along the entire surface of the blade. These circumstances present significant problems to one constructing a viable ceramic blade assembly since ceramics are brittle and have little capacity for withstanding mechanical or thermally induced tensile stresses while at the same time at elevated turbine inlet temperatures even the most advanced high-strength metals require complex and costly cooling methods.